1. Field of the Invention
The present invention relates to an imaging device, an imaging device body, and a lens barrel that include an apodization filter.
2. Description of the Related Art
There is known an imaging device that includes an apodization filter (hereinafter, referred to as an APD filter) having optical characteristics in which light transmittance is reduced as a distance from an optical axis is increased (see JP2005-062733A corresponding to U.S. Pat. No. 7,099,555). The APD filter reduces the amount of light around only a blurred image (spotlight blurring or the like), which is out of focus, without reducing the amount of light around an image plane. Accordingly, the APD filter realizes a beautiful blur by applying gradation to the profile of a blurred image.
Since the APD filter has the above-mentioned optical characteristics, a light-reducing effect becomes great as the diaphragm value (F number) approaches an open value. For this reason, in JP2005-062733A, a substantial diaphragm value (T number), which is obtained by correcting a diaphragm value on the basis of the optical characteristics of the APD filter, is obtained and the determination of an imaging exposure (an imaging diaphragm value and an imaging shutter speed) is performed on the basis of the T number.
Further, an imaging device, such as a digital camera, has an auto-focusing function. Auto-focusing is a method that moves a focus lens by a predetermined search interval and detects a focus position on the basis of high-frequency components of an imaging signal obtained for each search interval. JP2009-048136A (corresponding to U.S. Pat. No. 7,911,527) proposes a method of determining a search interval on the basis of the depth of focus, which is a parameter depending on a diaphragm value (F number) in the auto-focusing.
A method of determining the search interval of the auto-focusing on the basis of the depth of focus as disclosed in JP2009-048136A is also considered in the imaging device including the APD filter disclosed in JP2005-062733A. That is, since the determination of an imaging exposure is performed as described above in the imaging device including the APD filter, the diaphragm value (F number) is corrected to a substantial diaphragm value (T number). Accordingly, the depth of focus based on the T number is also used for the determination of the search interval. Since the T number is larger than the F number, the depth of focus, which is larger than the depth of focus obtained in a case in which the imaging device does not include the APD filter, is calculated in the imaging device including the APD filter. Accordingly, the search interval is determined as a large value.
However, since the APD filter is a kind of neutral density filter and actually does not change the depth of focus, the accuracy of detection of a focus position deteriorates due to an increase in the search interval. Since the T number becomes larger than the F number as approaching an open value, the accuracy of detection of a focus position deteriorates as the diaphragm value approaches the open value.
On the other hand, in the event that the depth of focus is determined on the basis of not the T number but the F number in order to prevent the deterioration of the accuracy of detection of a focus position and the search interval is determined using this depth of focus, the search interval is reduced as the F number approaches an open value. For this reason, the detection speed of a focus position is reduced. The detection speed of a focus position is reduced as the F number approaches the open value.